Nitrogen production by pressure swing adsorption (PSA) for industrial gas usage has become a significant share of the industrial gas market in recent years. However, this growth has been limited to a fairly well defined range of flow rates and purities over which the technology of pressure swing adsorption has been perceived by those skilled in the art to be amenable. A particular crucial limitation has been that the performance of traditional nitrogen pressure swing adsorption designs, both in power and capital costs, falls off sharply at purities of nitrogen greater than 99.9% by volume. For this reason, markets demanding greater than 99.9% by volume nitrogen purity have not been penetrated by pressure swing adsorption technologies to any great extent, and have largely been served by liquid nitrogen or liquid-assist nitrogen systems. However, the costs of this industrial nitrogen is relatively high, particularly in Japan and in the rapidly-developing markets in Latin America and Asia. There is growing demand for an economically competitive, non-cryogenic high purity nitrogen technology.
The existing technology for the production of non-cryogenic industrial nitrogen at purities of less than 0.1% oxygen include deoxo systems, vacuum swing adsorption systems, various modified pressure swing adsorption swing systems and various improvements on adsorbents used in such systems. However, despite these attempts to produce high purity industrial nitrogen such as is available from cryogenic large scale air separation systems, the industry has not provided a non-cryogenic nitrogen production process having high purities in excess of 99.9% nitrogen by volume with low capital cost and low power requirements. Various attempts have been made to produce non-cryogenic high purity nitrogen, such as recited below.
U.S. Pat. No. 5,176,722 discloses a pressure swing adsorption method which may use vacuum evacuation to produce nitrogen from air. The process recites the substantial equalization of pressures between two cyclically operated beds. Repressurization after equalization is conducted with product gas and feed gas mixture sequentially. During equalization, gas from the outlet of the high pressure bed is introduced to the inlet of a low pressure bed or optionally, gas from the outlet of high pressure bed is introduced simultaneously to the inlet and outlet of a low pressure bed. The simultaneous pressure equalization of both the inlet and outlet of the low pressure bed can be conducted for a portion or all of the pressure equalization step. Purities of this process are recited to be in the 90 to 99.9% range.
U.S. Pat. No. 5,090,973 discloses a pressure swing adsorption process for separating nitrogen from oxygen in air, in which a bed finishing adsorption can be evacuated and purged using a product-type gas having a higher purity than product generated by the adsorption bed itself. This higher purity product-type gas is produced by further purification using cryogenic or catalytic means or a gas sourced from a liquid source originating from other than the adsorption system itself. Simultaneous top and bottom pressure equalization is disclosed. As a result of the higher purity purge gas than product produced from the process being utilized for regeneration, product purities of 95 to 99.99% by volume nitrogen are identified as achievable.
U.S. Pat. No. 4,925,461 discloses an air separation process producing 99.99% by volume nitrogen, using either ambient depressurization or vacuum level depressurization and evacuation. Pressure equalizations are described as being short of full equalization and the equalization is conducted at the feed to feed ends of two beds while also being conducted at the outlet to outlet ends of the beds, wherein the gas transfer ratio of the feed ends to the outlet ends during pressure equalization is in the range of 3 to 70%.
Japanese Kokai 64-56113 similarly discloses a nitrogen pressure swing adsorption process with simultaneous pressure equalizations between the inlet and outlets of two beds, in which the flow rate across the inlets in comparison to the outlets is 3 to 70%. The patent advocates conducting the top and bottom gas transfer communications simultaneously to markedly decrease abrasion of the adsorbent particles. Oxygen impurity levels are reduced by this process down to 0.064% oxygen.
Japanese Kokai 63-79714 discloses a process for producing 99.99% nitrogen from air using a configuration of three adsorption beds, which during the regeneration are coupled in series.
Although the industry has created high purity nitrogen in excess of 99.9% nitrogen by volume in various pressure swing adsorption and vacuum swing adsorption processes, these processes have required unique sources of purge gas, additional purification technologies, additional capital costs for complex piping schemes and high power and overall capital costs. The present invention solves the problem of producing high purity nitrogen in excess of 99.9% nitrogen by volume in a low capital, low power process which minimizes the complexity of process operation. The present invention will be set forth below.